An electrophotographic image duplicating machine, for example, has functional modules including a control panel and an automatic document feed module in addition to an image reproducing module for the reproduction of duplicate images. Each of the functional modules operates under the control of a subsidiary system control unit exclusively associated with the particular functional module. The respective subsidiary system control units associated with the individual functional modules in turn are under the control of a single main system control unit which is operative not only to control each of the subsidiary system control units but to coordinate the operation to be performed by two or more of the functional modules.
For exchange of data between the main system control unit and each of the subsidiary system control units in this type of image forming apparatus, there may be used a data communication network of either the leased transmission line type or the common interface bus type in a data communication control system for the image forming apparatus. In a data communication control system using a data communication network of the leased transmission line type, each of the subsidiary system control units is electrically coupled to the main system control unit through transmission lines proper to the particular subsidiary system control unit so that each of the subsidiary system control units is permitted to communicate with the main system control unit independently of the other subsidiary system control units. A data communication network of this type however has a drawback encountered when an additional functional module is to be coupled to the apparatus at the user's option. To enable the main system control unit to be in control of the additional functional module, additional data transmission lines for the additional functional module must be provided and connected to the main system control unit.
A drawback of this nature is eliminated in a data communication control system using a full-duplex communication network of the common interface bus type in which the respective subsidiary system control units are coupled to the main system control unit through an interface bus common to the individual subsidiary system control units. A particular address is assigned to each of the plurality of subsidiary system control units so that the individual subsidiary system control units are one after another accessed by the main system control unit with the addresses for the subsidiary system control units designated successively. A known example of a data communication network of this common interface bus type is taught in Japanese Provisional Patent Publication (Kokai) No. 59-127133.
In an apparatus having a plurality of functional modules including those which may be selectively added to the apparatus at the user's option, it sometimes happens that any one of the modules is not assembled to the apparatus and is thus unavailable. It may further happen that the functional module which has thus been unavailable is installed into the apparatus in addition to the existing functional modules after the apparatus is put to use or during operation of the apparatus. If this happens in an apparatus including a data communication network of the common interface bus type, it is required that the subsidiary system control unit in control of the functional module newly added to the apparatus be coupled in both hardware and software to the main system control unit of the system. An object of the present invention is to realize such a function in a data communication control system using a full-duplex communication network of the common interface bus type.
Each of the main and subsidiary control units of a data communication control system of the described character is typically implemented by a semiconductor microprocessor. Where the control system is incorporated in an image forming apparatus having, for example, a control panel, an image reproducing module and an automatic document feed module, the interface between each of these functional modules and the data communication control system is dictated by the microprocessor forming part of the main system control unit. Where the image forming apparatus is equipped with an increased number of functional modules, the microprocessor of the main system control unit must bear an increased amount of burden to be completely predominant over the operation of the individual functional modules. Another object of the present invention is to provide a useful solution to this problem.